less

SYNOPSIS

less -?
less --help
less -V
less --version
less [-[+]aBcCdeEfgGiImMnNqQrsSuUVwX] [-b bufs]
	 [-h lines] [-j line] [-k keyfile]
	 [-{oO} logfile] [-p pattern] [-P prompt]
	 [-t tag] [-T tagsfile] [-x tab] [-y lines]
	 [-[z] lines] [+[+]cmd] [--] [filename]...

The less(1) utility is similar to more(1), but it allows backward movement in the file as well as forward movement. In addition, less(1) does not have to read the entire input file before starting, so with large input files it starts up faster than text editors like vi(1). The less(1) utility uses termcap (or terminfo on some systems), to run on a variety of terminals. There is even limited support for hardcopy terminals. (On a hardcopy terminal, lines that should be printed at the top of the screen are prefixed with a caret symbol (^).)

Commands are based on both more(1) and vi(1). Commands can be preceded by a decimal number, called N in the descriptions below. The number is used by some commands, as indicated.

COMMANDS

In the following descriptions, ^X means CTRL-X. ESC stands for the ESCAPE key; for example ESC-v means the two character sequence "ESCAPE", then "v".

h or H

Help: display a summary of these commands. This is a useful command to remember.

SPACE or ^V or f or ^F

Scroll forward N lines, default one window (see option -z below). If N is more than the screen size, only the final screenful is displayed. Note that some systems use ^V as a special literalization character.

z

Like SPACE, but if N is specified, it becomes the new window size.

ESC-SPACE

Like SPACE, but scrolls a full screenful, even if it reaches end-of-file in the process.

RETURN or ^N or e or ^E or j or ^J

Scroll forward N lines, default 1. The entire N lines are displayed, even if N is more than the screen size.

d or ^D

Scroll forward N lines, default one half of the screen size. If N is specified, it becomes the new default for subsequent d and u commands.

b or ^B or ESC-v

Scroll backward N lines, default one window (see option -z below). If N is more than the screen size, only the final screenful is displayed.

w

Like ESC-v, but if N is specified, it becomes the new window size.

y or ^Y or ^P or k or ^K

Scroll backward N lines, default 1. The entire N lines are displayed, even if N is more than the screen size. Note that some systems use ^Y as a special job-control character.

u or ^U

Scroll backward N lines, default one half of the screen size. If N is specified, it becomes the new default for subsequent d and u commands.

ESC-) or RIGHTARROW

Scroll horizontally right N characters, default 8. This behaves best if you also set the -S option (chop lines). Note that if you want to enter a number N, you must use ESC-), not RIGHTARROW, because the arrow is taken to be a line-editing command (see the LINE EDITING section).

ESC-( or LEFTARROW

Scroll horizontally left N characters, default 8.

r or ^R or ^L

Repaint the screen.

R

Repaint the screen, discarding any buffered input. Useful if the file is changing while it is being viewed.

F

Scroll forward, and keep trying to read when the end of file is reached. Normally, this command would be used when already at the end of the file. It is a way to monitor the tail of a file that is growing while it is being viewed. (The behavior is similar to the tail -f command.)

g or < or ESC-<

Go to line N in the file, default 1 (beginning of file). (This might be slow if N is large.)

G or > or ESC->

Go to line N in the file, default the end of the file. (This might be slow if N is large, or if N is not specified and standard input, rather than a file, is being read.)

p or %

Go to a position N percent into the file. N should be between 0 and 100.

{

If a left curly bracket appears in the top line displayed on the screen, the { command will go to the matching right curly bracket. The matching right curly bracket is positioned on the bottom line of the screen. If there is more than one left curly bracket on the top line, a number N can be used to specify the N-th bracket on the line.

}

If a right curly bracket appears in the bottom line displayed on the screen, the } command will go to the matching left curly bracket. The matching left curly bracket is positioned on the top line of the screen. If there is more than one right curly bracket on the top line, a number N can be used to specify the N-th bracket on the line.

(

Like {, but applies to parentheses rather than curly brackets.

)

Like }, but applies to parentheses rather than curly brackets.

[

Like {, but applies to square brackets rather than curly brackets.

]

Like }, but applies to square brackets rather than curly brackets.

ESC-^F

Followed by two characters, acts like {, but uses the two characters as open and close brackets, respectively. For example, ESC ^F < > could be used to go forward to the > that matches the < in the top displayed line.

ESC-^B

Followed by two characters, acts like }, but uses the two characters as open and close brackets, respectively. For example, "ESC ^B < >" could be used to go backward to the < that matches the > in the bottom displayed line.

m

Followed by any lowercase letter, marks the current position with that letter.

'

(Single quote.) Followed by any lowercase letter, returns to the position that was previously marked with the letter. Followed by another single quote, returns to the position at which the last "large" movement command was executed. Followed by a ^ or $, jumps to the beginning or end of the file respectively. Marks are preserved when a new file is examined, so the ' command can be used to switch between input files.

^X^X

Same as single quote.

/pattern

Search forward in the file for the occurrence of the line, indicated by N, that contains the pattern. N defaults to 1. The pattern is a regular expression, as recognized by ed(1). The search starts at the second line displayed (but see the -a and -j options, which change this).

Certain characters are special if entered at the beginning of the pattern; they modify the type of search rather than becoming part of the pattern:

^N or !: Search for lines that do not match the pattern.
^E or *: Search multiple files. That is, if the search reaches the end of the current file without finding a match, the search continues in the next file in the command-line list.
^F or @: Begin the search at the first line of the first file in the command-line list, regardless of what is currently displayed on the screen or the settings of the -a or -j options.
^K: Highlight any text that matches the pattern on the current screen, but do not move to the first match (keep current position).
^R: Do not interpret regular expression metacharacters; that is, do a simple textual comparison.

?pattern

Search backward in the file for the occurrence of the line, indicated by N, containing the pattern. The search starts at the line immediately before the top line displayed.

Certain characters are special as in the / command:

^N or !: Search for lines that do not match the pattern.
^E or *: Search multiple files. That is, if the search reaches the beginning of the current file without finding a match, the search continues in the previous file in the command line list.
^F or @: Begin the search at the last line of the last file in the command-line list, regardless of what is currently displayed on the screen or the settings of the -a or -j options.
^K: As in forward searches.
^R: As in forward searches.

ESC-/pattern

Same as "/*".

ESC-?pattern

Same as "?*".

n

Repeat previous search for the occurrence of the line, indicated by N, containing the last pattern. If the previous search was modified by ^N, the search is made for the occurrence of the line indicated by N not containing the pattern. If the previous search was modified by ^E, the search continues in the next (or previous) file if not satisfied in the current file. If the previous search was modified by ^R, the search is done without using regular expressions. There is no effect if the previous search was modified by ^F or ^K.

N

Repeat previous search, but in the reverse direction.

ESC-n

Repeat previous search, but crossing file boundaries. The effect is as if the previous search were modified by *.

ESC-N

Repeat previous search, but in the reverse direction and crossing file boundaries.

ESC-u

Undo search highlighting. Turn off highlighting of strings matching the current search pattern. If highlighting is already off because of a previous ESC-u command, turn highlighting back on. Any search command will also turn highlighting back on. (Highlighting can also be disabled by toggling the -G option; in that case search commands do not turn highlighting back on.)

:e [filename]

Examine a new file. If the file name is missing, the "current" file (see the :n and :p commands late in this topic) from the list of files in the command line is reexamined. A percent sign (%) in the file name is replaced by the name of the current file. A pound sign (#) is replaced by the name of the previously examined file. Two consecutive percent signs are simply replaced with a single percent sign. This allows you to enter a file name that contains a percent sign in the name. Similarly, two consecutive pound signs are replaced with a single pound sign. The file name is inserted into the command-line list of files so that it can be seen by subsequent :n and :p commands. If the file name consists of several files, they are all inserted into the list of files and the first one is examined. If the file name contains one or more spaces, the entire file name should be enclosed in double quotes ("). (See also the - option).

^X^V or E

Same as :e. Note that some systems use ^V as a special literalization character. On such systems, you might not be able to use ^V.

:n

Examine the next file (from the list of files given in the command line). If a number, N, is specified, the next occurrence of the file indicated by N is examined.

:p

Examine the previous file in the command-line list. If a number N is specified, the occurrence of the file indicated by N is examined.

:x

Examine the first file in the command-line list. If a number N is specified, the occurrence in the list of the file indicated by Nis examined.

= or ^G or :f

Prints some information about the file being viewed, including its name and the line number, and byte offset of the bottom line being displayed. If possible, it also prints the length of the file, the number of lines in the file, and the percent of the file above the last displayed line.

-

Followed by one of the command-line option letters (discussed later in this topic), this will change the setting of that option and print a message describing the new setting. If the option letter has a numeric value (such as -b or -h), or a string value (such as -P or -t), a new value can be entered after the option letter. If no new value is entered, a message describing the current setting is printed and nothing is changed.

-+

Followed by one of the command-line option letters, this will reset the option to its default setting and print a message describing the new setting. (The "-+X" command does the same thing as "-+X" on the command line.) This does not work for string-valued options.

--

Followed by one of the command-line option letters (see below), this will reset the option to the "opposite" of its default setting and print a message describing the new setting. (The "--X" command does the same thing as "-X" on the command line.) This does not work for numeric or string-valued options.

_

(Underscore.) Followed by one of the command-line option letters, this will print a message describing the current setting of that option. The setting of the option is not changed.

+cmd

Causes the specified cmd to be executed each time a new file is examined. For example, +G causes less(1) to initially display each file starting at the end rather than the beginning.

V

Prints the version number of less(1) being run.

q or Q or :q or :Q or ZZ

Exits less(1).

The following four commands may or may not be valid, depending on your particular installation.

v: Invokes an editor to edit the file currently being viewed. The editor is taken from the environment variable VISUAL if defined, or EDITOR if VISUAL is not defined. It defaults to "vi" if neither VISUAL nor EDITOR is defined. See the discussion of LESSEDIT in the section on PROMPTS later in this topic.
! shell-command: Invokes a shell to run the shell-command given. A percent sign (%) in the command is replaced by the name of the current file. A pound sign (#) is replaced by the name of the previously examined file. Two exclamation marks (!!) repeats the last shell command. A single exclamation mark (!) with no shell command simply invokes a shell. On UNIX systems, the shell is taken from the environment variable SHELL, or defaults to "sh". On MS-DOS and OS/2 systems, the shell is the normal command processor.
| m shell-command: The m represents any mark letter. Pipes a section of the input file to the given shell command. The section of the file to be piped is between the first line on the current screen and the position marked by the letter.The m can also be ^ or $ to indicate beginning or end of file respectively. If m is a period (.) or newline, the current screen is piped.
s filename: Save the input to a file. This only works if the input is a pipe, not an ordinary file.

OPTIONS

Most options can be changed while less(1) is running, through the "-" command.

Options are also taken from the environment variable LESS. For example, to avoid typing less -options ... each time less(1) is invoked, you might tell csh(1):

setenv LESS "-options"

or, if you use sh(1):

LESS="-options"; export LESS

On MS-DOS, you do not need the quotation marks, but you should replace any percent signs in the options string with double percent signs.

The environment variable is parsed before the command line, so command -ine options override the LESS environment variable. If an option appears in the LESS variable, it can be reset to its default on the command line by beginning the command line option with "-+".

For options like -P, which take a following string, a dollar sign ($) can be used to signal the end of the string.

-?: This option displays a summary of the commands accepted by less(1) (the same as the h command). (Depending on how your shell interprets the question mark, it may be necessary to quote the question mark, thus: "-\?".)
--help: Same as -?.
-a: Causes searches to start after the last line displayed on the screen, thus skipping all lines displayed on the screen. By default, searches start at the second line on the screen (or after the last found line; see the -j option).
-bn: Specifies the number of buffers less(1) will use for each file. Buffers are 1K, and by default 10 buffers are used for each file (unless the file is a pipe; see the -B option). The number n specifies a different number of buffers to use.
-B: By default, when data is read from a pipe, buffers are allocated automatically as needed. If a large amount of data is read from the pipe, this can cause a large amount of memory to be allocated. The -B option disables this automatic allocation of buffers for pipes, so that only the number of buffers specified by the -b option are used. Note that the use of -B can result in erroneous display because only the most recently viewed part of the file is kept in memory; earlier data is lost.
-c: Causes full-screen repaints to be painted from the top line down. By default, full-screen repaints are done by scrolling from the bottom of the screen.
-C: The -C option is like -c, but the screen is cleared before it is repainted.
-d: The -d option suppresses the error message normally displayed if the terminal is dumb; that is, lacks some important capability, such as the ability to clear the screen or scroll backward. The -d option does not otherwise change the behavior of less(1) on a dumb terminal).
Causes less(1) to automatically exit the second time it reaches end-of-file. By default, the only way to exit less(1) is through the q command.
-E: Causes less(1) to automatically exit the first time it reaches end-of-file.
-f: Forces non-regular files to be opened. (A non-regular file is a directory or a device special file.) Also suppresses the warning message when a binary file is opened. By default, less(1) will refuse to open non-regular files.
-g: Normally, less(1) will highlight ALL strings that match the last search command. The -g option changes this behavior to highlight only the particular string that was found by the last search command. This can cause less(1) to run somewhat faster than the default.
-G: The -G option suppresses all highlighting of strings found by search commands.
-hn: Specifies a maximum number of lines to scroll backward. If it is necessary to scroll backward more than n lines, the screen is repainted in a forward direction instead. (If the terminal does not have the ability to scroll backward, -h0 is implied.)
-i: Causes searches to ignore case; that is, uppercase and lowercase are considered identical. This option is ignored if any uppercase letters appear in the search pattern; in other words, if a pattern contains uppercase letters, that search does not ignore case.
-I: Like -i, but searches ignore case even if the pattern contains uppercase letters.
-jn: Specifies a line on the screen where the "target" line is to be positioned. A target line is the object of a text search, tag search, jump to a line number, jump to a file percentage, or jump to a marked position. The screen line is specified by a number: the top line on the screen is 1, the next is 2, and so on. The number can be negative to specify a line relative to the bottom of the screen: the bottom line on the screen is -1, the second to the bottom is -2, and so on. If the -j option is used, searches begin at the line immediately after the target line. For example, if "-j4" is used, the target line is the fourth line on the screen, so searches begin at the fifth line on the screen.
-kfilename: Causes less(1) to open and interpret the named file as a lesskey(1) file. Multiple -k options may be specified. If the LESSKEY environment variable is set, or if a lesskey file is found in a standard place (see KEY BINDINGS), it is also used as a lesskey(1) file.
-m: Causes less(1) to prompt verbosely (like more(1)), with the percent into the file. By default, less(1) prompts with a colon.
-M: Causes less(1) to prompt even more verbosely than more(1).
-n: Suppresses line numbers. The default (to use line numbers) can cause less(1) to run more slowly in some cases, especially with a very large input file. Suppressing line numbers with the -n option will avoid this problem. Using line numbers means that the line number will be displayed in the verbose prompt and in the = command, and the v command will pass the current line number to the editor (see also the discussion of LESSEDIT in PROMPTS later in this topic).
-N: Causes a line number to be displayed at the beginning of each line in the display.
-ofilename: Causes less(1) to copy its input to the named file as it is being viewed. This applies only when the input file is a pipe, not an ordinary file. If the file already exists, less(1) will ask for confirmation before overwriting it.
-Ofilename: The -O option is like -o, but it will overwrite an existing file without asking for confirmation.
If no log file has been specified, the -o and -O options can be used from within less(1) to specify a log file. Without a file name, they will simply report the name of the log file. The "s" command is equivalent to specifying -o from within less(1).
-ppattern: The -p option on the command line is equivalent to specifying "+/pattern"; that is, it tells less(1) to start at the first occurrence of pattern in the file.
-Pprompt: Provides a way to tailor the three prompt styles to your own preference. This option would normally be put in the LESS environment variable, rather than being typed in with each less(1) command. Such an option must either be the last option in the LESS variable, or be terminated by a dollar sign. -Ps followed by a string changes the default (short) prompt to that string. -Pm changes the medium (-m) prompt. -PM changes the long (-M) prompt. -Ph changes the prompt for the help screen. -P= changes the message printed by the = command. All prompt strings consist of a sequence of letters and special escape sequences. See the section on PROMPTS for more details.
-q: Causes moderately "quiet" operation: the terminal bell is not rung if an attempt is made to scroll past the end of the file or before the beginning of the file. If the terminal has a "visual bell", it is used instead. The bell will ring on certain other errors, such as typing an invalid character. The default is to ring the terminal bell in all such cases.
-Q: Causes totally "quiet" operation: the terminal bell is never rung.
-r: Causes "raw" control characters to be displayed. The default is to display control characters using the caret notation; for example, a control-A (octal 001) is displayed as "^A". When the -r option is used, less(1) cannot keep track of the actual appearance of the screen (because this depends upon how the screen responds to each type of control character). Thus, various display problems can result, such as long lines being split in the wrong place.
-s: Causes consecutive blank lines to be squeezed into a single blank line. This is useful when viewing nroff(1) output.
-S: Causes lines longer than the screen width to be chopped rather than folded. That is, the remainder of a long line is simply discarded. The default is to fold long lines; that is, display the remainder on the next line.
-ttag: The -t option, followed immediately by a tag, will edit the file containing that tag. For this to work, there must be a file called tags in the current directory, which was previously built by the ctags(1) command. This option can also be specified from within less(1) (using the - command) as a way of examining a new file. The command ":t" is equivalent to specifying -t from within less(1).
-Ttagsfile: Specifies a tags file to be used instead of "tags".
-u: Causes backspaces and carriage returns to be treated as printable characters; that is, they are sent to the terminal when they appear in the input.
-U: Causes backspaces, tabs, and carriage returns to be treated as control characters; that is, they are handled as specified by the -r option.
By default, if neither -u nor -U is given, backspaces that appear adjacent to an underscore character are treated specially: the underlined text is displayed using the terminal's hardware underlining capability. Also, backspaces that appear between two identical characters are treated specially: the overstruck text is printed using the terminal's hardware boldface capability. Other backspaces are deleted, along with the preceding character. Carriage returns immediately followed by a newline are deleted. Other carriage returns are handled as specified by the -r option. Text that is overstruck or underlined can be searched for if neither -u nor -U is in effect.
-V: Displays the version number of less(1).
--version: Same as -V.
-w: Causes blank lines to be used to represent lines past the end of the file. By default, a tilde character (~) is used.
-xn: Sets tab stops every n positions. The default for 2n is 8.
-X: Disables sending the termcap initialization and deinitialization strings to the terminal. This is sometimes desirable if the deinitialization string does something unnecessary, like clearing the screen.
-yn: Specifies a maximum number of lines to scroll forward. If it is necessary to scroll forward more than n lines, the screen is repainted instead. You can use the -c or -C option to repaint from the top of the screen. By default, any forward movement causes scrolling.
-[z]n: Changes the default scrolling window size to n lines. The default is one screenful. The z and w commands can also be used to change the window size. The "z" can be omitted for compatibility with more(1). If the number n is negative, it indicates n lines fewer than the current screen size. For example, if the screen is 24 lines, -z-4 sets the scrolling window to 20 lines. If the screen is resized to 40 lines, the scrolling window automatically changes to 36 lines.
-: Changes the file name quoting character. This might be necessary if you are trying to name a file that contains both spaces and quote characters. Followed by a single character, this changes the quote character to that character. File names containing a space should then be surrounded by that character rather than by double quotes. Followed by two characters, changes the open quote to the first character, and the close quote to the second character. File names containing a space should then be preceded by the open quote character and followed by the close quote character. Note that even after the quote characters are changed, this option remains -" (a dash followed by a double quote).
--: The command line argument -- marks the end of option arguments. Any arguments following this are interpreted as file names. This can be useful when viewing a file whose name begins with a "-" or "+".
+: If a command line option begins with +, the remainder of that option is taken to be an initial command to less(1). For example, +G tells less(1) to start at the end of the file rather than the beginning, and +/xyz tells it to start at the first occurrence of "xyz" in the file. As a special case, +<number> acts like +<number>g; that is, it starts the display at the specified line number (however, see the caveat under the "g" command discussed previously in this topic). If the option starts with ++, the initial command applies to every file being viewed, not just the first one. The + command described previously may also be used to set (or change) an initial command for every file.

LINE EDITING

When entering command line at the bottom of the screen (for example, a file name for the :e command, or the pattern for a search command), certain keys can be used to manipulate the command line. Most commands have an alternate form in brackets ([]) that can be used if a key does not exist on a particular keyboard. (The bracketed forms do not work in the MS-DOS version.) Any of these special keys can be entered literally by preceding it with the "literal" character, either ^V or ^A. A backslash can also be entered literally by entering two backslashes.

LEFTARROW [ ESC-h ]: Move the cursor one space to the left.
RIGHTARROW [ ESC-l ]: Move the cursor one space to the right.
^LEFTARROW [ ESC-b or ESC-LEFTARROW ]: (That is, CONTROL and LEFTARROW simultaneously.) Move the cursor one word to the left.
^RIGHTARROW [ ESC-w or ESC-RIGHTARROW ]: (That is, CONTROL and RIGHTARROW simultaneously.) Move the cursor one word to the right.
HOME [ ESC-0 ]: Move the cursor to the beginning of the line.
END [ ESC-$ ]: Move the cursor to the end of the line.
BACKSPACE: Delete the character to the left of the cursor, or cancel the command if the command line is empty.
DELETE or [ ESC-x ]: Delete the character under the cursor.
^BACKSPACE [ ESC-BACKSPACE ]: (That is, CONTROL and BACKSPACE simultaneously.) Delete the word to the left of the cursor.
^DELETE [ ESC-X or ESC-DELETE ]: (That is, CONTROL and DELETE simultaneously.) Delete the word under the cursor.
UPARROW [ ESC-k ]: Retrieve the previous command line.
DOWNARROW [ ESC-j ]: Retrieve the next command line.
TAB: Complete the partial file name to the left of the cursor. If it matches more than one file name, the first match is entered into the command line. Repeated TABs will cycle through the other matching file names. If the completed file name is a directory, a "/" is appended to the file name. (On MS-DOS systems, a "\" is appended.) The environment variable LESSSEPARATOR can be used to specify a different character to append to a directory name.
BACKTAB [ ESC-TAB ]: Like, TAB, but cycles in the reverse direction through the matching file names.
^L: Complete the partial file name to the left of the cursor. If it matches more than one file name, all matches are entered into the command line (if they fit).
^U (UNIX) or ESC (MS-DOS): Delete the entire command line, or cancel the command if the command line is empty. If you have changed your line-kill character in UNIX to something other than ^U, that character is used instead of ^U.

KEY BINDINGS

You can define your own less(1) commands by using the program lesskey(1) to create a lesskey file. This file specifies a set of command keys and an action associated with each key. You can also use lesskey(1) to change the line-editing keys (see LINE EDITING), and to set environment variables. If the environment variable LESSKEY is set, less(1) uses that as the name of the lesskey file. Otherwise, less(1) looks in a standard place for the lesskey file: On UNIX systems, less(1) looks for a lesskey file called $HOME/.less. On MS-DOS systems, less(1) looks for a lesskey file called $HOME/_less; and if it is not found there, then looks for a lesskey file called _less in any directory specified in the PATH environment variable. On OS/2 systems, less(1) looks for a lesskey file called $HOME/less.ini, and if it is not found, then looks for a lesskey file called less.ini in any directory specified in the INIT environment variable, and if it not found there, then looks for a lesskey file called less.ini in any directory specified in the PATH environment variable. See the lesskey(1) manual page for more details.

INPUT PREPROCESSOR

You can define an input preprocessor for less(1). Before less(1) opens a file, it first gives your input preprocessor a chance to modify the way the contents of the file are displayed. An input preprocessor is simply an executable program (or shell script), that writes the contents of the file to a different file, called the replacement file. The contents of the replacement file are then displayed in place of the contents of the original file. It will appear to the user, however, as if the original file is opened; that is, less(1) will display the original file name as the name of the current file.

An input preprocessor receives one command-line argument, the original file name, as entered by the user. It should create the replacement file, and when finished, print the name of the replacement file to its standard output. If the input preprocessor does not output a replacement file name, less(1) uses the original file, as normal. The input preprocessor is not called when viewing standard input. To set up an input preprocessor, set the LESSOPEN environment variable to a command line, which will invoke your input preprocessor. This command line should include one occurrence of the string "%s", which will be replaced by the file name when the input preprocessor command is invoked.

When less(1) closes a file opened in such a way, it will call another program, called the input postprocessor, which can perform any desired clean-up action (such as deleting the replacement file created by LESSOPEN). This program receives two command-line arguments, the original file name as entered by the user, and the name of the replacement file. To set up an input postprocessor, set the LESSCLOSE environment variable to a command line which will invoke your input postprocessor. It can include two occurrences of the string "%s"; the first is replaced with the original name of the file and the second with the name of the replacement file, which was output by LESSOPEN.

For example, on many UNIX systems, these two scripts will allow you to keep files in compressed format, but still let less(1) view them directly:

lessopen.sh:

#! /bin/sh
case "$1" in
*.Z) uncompress
-c
$1  >/tmp/less.$$  2>/dev/null
	 if [
-s
/tmp/less.$$ ]; then
		echo /tmp/less.$$
	 else
		rm
-f
/tmp/less.$$
	 fi
	 ;;
esac

lessclose.sh:

#! /bin/sh
rm $2

To use these scripts, put them both where they can be executed and set LESSOPEN=lessopen.sh %s, and LESSCLOSE=lessclose.sh %s %s. More complex LESSOPEN and LESSCLOSE scripts can be written to accept other types of compressed files, and so on.

It is also possible to set up an input preprocessor to pipe the file data directly to less(1), rather than putting the data into a replacement file. This eliminates the need to decompress the entire file before starting to view it. An input preprocessor that works this way is called an input pipe. An input pipe, instead of writing the name of a replacement file on its standard output, writes the entire contents of the replacement file on its standard output. If the input pipe does not write any characters on its standard output, there is no replacement file and less(1) uses the original file, as normal. To use an input pipe, make the first character in the LESSOPEN environment variable a vertical bar (|) to signify that the input preprocessor is an input pipe.

For example, on many UNIX systems, this script will work like the previous example scripts:

lesspipe.sh:

#! /bin/sh
case "$1" in
*.Z) uncompress
-c
$1  2>/dev/null
	 ;;
esac

To use this script, put it where it can be executed and set LESSOPEN=|lesspipe.sh %s. When an input pipe is used, a LESSCLOSE postprocessor can be used, but it is usually not necessary since there is no replacement file to clean up. In this case, the replacement file name passed to the LESSCLOSE postprocessor is "-".

NATIONAL CHARACTER SETS

There are three types of characters in the input file:

Normal characters: Can be displayed directly to the screen.
Control characters: Should not be displayed directly, but are expected to be found in ordinary text files (such as backspace and tab).
Binary characters: Should not be displayed directly and are not expected to be found in text files.

A "character set" is simply a description of the characters that are to be considered normal, control, and binary. The LESSCHARSET environment variable can be used to select a character set. Possible values for LESSCHARSET are:

ascii: The default character set. BS, TAB, NL, CR, and formfeed are control characters, all chars with values between 127 and 255 are binary, and all others are normal.
latin1: Selects the International Standards Organization (ISO) 8859/1 character set. latin-1 is the same as ASCII, except characters between 161 and 255 are treated as normal characters.
dos: Selects a character set appropriate for MS-DOS.
koi8-r: Selects a Russian character set.
next: Selects a character set appropriate for NeXT computers.

In special cases, you might want to tailor less(1) to use a character set other than the ones definable by LESSCHARSET. In this case, the environment variable LESSCHARDEF can be used to define a character set. It should be set to a string where each character in the string represents one character in the character set. The character "." is used for a normal character, "c" for control, and "b" for binary. A decimal number may be used for repetition. For example, "bccc4b." would mean character 0 is binary; 1, 2, and 3 are control; 4, 5, 6, and 7 are binary; and 8 is normal. All characters after the last are taken to be the same as the last, so characters 9 through 255 would be normal. (This is an example, and does not necessarily represent any real character set.)

This table shows the value of LESSCHARDEF that is equivalent to each of the possible values for LESSCHARSET:

`ascii`	8bcccbcc18b95.b
`latin1`	8bcccbcc18b95.33b.
`dos`	8bcccbcc12bc5b95.b.
`koi8-r`	8bcccbcc18b95.b128.
`next`	8bcccbcc18b95.bb125.bb

If neither LESSCHARSET nor LESSCHARDEF is set, but your system supports the setlocale(3) interface, less(1) will use setlocale to determine the character set. setlocale is controlled by setting the LANG or LC_CTYPE environment variables.

Control and binary characters are displayed in standout (reverse video). Each such character is displayed in caret notation if possible (such as ^A for control-A). Caret notation is used only if inverting the 0100 bit results in a normal printable character. Otherwise, the character is displayed as a hex number in angle brackets. This format can be changed by setting the LESSBINFMT environment variable. LESSBINFMT can begin with a "*" and one character to select the display attribute: "*k" is blinking, "*d" is bold, "*u" is underlined, "*s" is standout, and "*n" is normal. If LESSBINFMT does not begin with a "*", normal attribute is assumed. The remainder of LESSBINFMT is a string that can include one printf-style escape sequence (a % followed by x, X, o, d, and so on). For example, if LESSBINFMT is "*u[%x]", binary characters are displayed in underlined hexadecimal surrounded by brackets. The default if no LESSBINFMT is specified is "*d<%X>".

PROMPTS

With the -P option, you can tailor the prompt to your preference. The string given to the -P option replaces the specified prompt string. Certain characters in the string are interpreted specially. The prompt mechanism is rather complicated to provide flexibility, but the ordinary user need not understand the details of constructing personalized prompt strings.

A percent sign followed by a single character is expanded according to what the following character is:

%bX

Replaced by the byte offset into the current input file. The b is followed by a single character (shown as X above) that specifies the line whose byte offset is to be used.

t: Use the byte offset of the top line in the display.
m: Use the middle line.
b: Use the bottom line.
B: Use the line just after the bottom line.
j: Use the "target" line, as specified by the -j option.

%B

Replaced by the size of the current input file.

%E

Replaced by the name of the editor (from the VISUAL environment variable, or the EDITOR environment variable if VISUAL is not defined). See the discussion of the LESSEDIT feature later in this topic.

%f

Replaced by the name of the current input file.

%i

Replaced by the index of the current file in the list of input files.

%lX

Replaced by the line number of a line in the input file. The line to be used is determined by the X, as with the %b option.

%L

Replaced by the line number of the last line in the input file.

%m

Replaced by the total number of input files.

%pX

Replaced by the percent into the current input file. The line used is determined by the X as with the %b option.

%s

Same as %B.

%t

Causes any trailing spaces to be removed. Usually used at the end of the string, but can appear anywhere.

%x

Replaced by the name of the next input file in the list.

If any item is unknown (for example, the file size if input is a pipe), a question mark is printed instead.

The format of the prompt string can be changed depending on certain conditions. A question mark (?) followed by a single character acts like an "IF": depending on the following character, a condition is evaluated. If the condition is true, any characters following the question mark and condition character, up to a period (.), are included in the prompt. If the condition is false, such characters are not included. A colon (B) appearing between the question mark and the period can be used to establish an "ELSE": any characters between the colon and the period are included in the string if and only if the IF condition is false. Condition characters (which follow a question mark) may be:

?a: True if any characters have been included in the prompt so far.
?bX: True if the byte offset of the specified line is known.
?B: True if the size of current input file is known.
?e: True if at end-of-file.
?f: True if there is an input file name (that is, if input is not a pipe).
?lX: True if the line number of the specified line is known.
?L: True if the line number of the last line in the file is known.
?m: True if there is more than one input file.
?n: True if this is the first prompt in a new input file.
?pX: True if the percent into the current input file of the specified line is known.
?s: Same as ?B.
?x: True if there is a next input file (that is, if the current input file is not the last one).

Any characters other than the special ones (question mark, colon, period, percent, and backslash) become literally part of the prompt. Any of the special characters can be included in the prompt literally by preceding it with a backslash.

Examples

?f%f:Standard input.

This prompt prints the file name, if known; otherwise, it prints the string "Standard input".

?f%f .?ltLine %lt:?pt%pt\e\e%:?btByte %bt:-...

This prompt prints the file name, if known. The file name is followed by the line number, if known; otherwise, the percent, if known; otherwise, the byte offset, if known. Otherwise, a dash is printed. Notice how each question mark has a matching period, and how the % after the %pt is included literally by escaping it with a backslash.

?n?f%f\ .?m(file\ %i\ of\ %m)\ ..?e(END)\ ?x-\ Next\\:\ %x..%t

This prints the file name if this is the first prompt in a file, followed by the "file N of N" message if there is more than one input file. Then, if end-of-file has been reached, the string "(END)" is printed, followed by the name of the next file, if there is one. Finally, any trailing spaces are truncated. This is the default prompt. For reference, here are the defaults for the other two prompts (-m and -M respectively). Each is broken into two lines here for readability only.

?n?f%f\ .?m(file\ %i\ of\ %m)\ ..?e(END)\ ?x-\ Next\\:\ %x.:
	 ?pB%pB\\%:byte\ %bB?s/%s...%t


?f%f\ .?n?m(file\ %i\ of\ %m)\ ..?ltline\ %lt?L/%L.\ :byte\ %bB?s/%s.\ .
	 ?e(END)\ ?x-\ Next\\:\ %x.:?pB%pB\\%..%t

Following is the default message produced by the = command:

?f%f\ .?m(file\ %i\ of\ %m)\ .?ltline\ %lt?L/%L.\ .
	 byte\ %bB?s/%s.\ ?e(END)\ :?pB%pB\\%..%t

The prompt expansion features are also used for another purpose: if an environment variable LESSEDIT is defined, it is used as the command to be executed when the v command is invoked. The LESSEDIT string is expanded in the same way as the prompt strings. The default value for LESSEDIT is:

%E\ ?lm+%lm.\ %f

This expands to the editor name, followed by a + and the line number, followed by the file name. If your editor does not accept the "+linenumber" syntax, or has other differences in invocation syntax, the LESSEDIT variable can be changed to modify this default.

SECURITY

When the environment variable LESSSECURE is set to 1, less(1) runs in a "secure" mode. This means that the following features are disabled:

!: The shell command
|: The pipe command
:e: The examine command
v: The editing command
s -o: Log files
-k: Use of lesskey files
-t: Use of tags files; Metacharacters in file names, such as *; File-name completion (TAB, .)

The less(1) utility can also be compiled to be in "secure" mode permanently.

ENVIRONMENT VARIABLES

Environment variables can be specified either in the system environment, as usual, or in a lesskey(1) file.

COLUMNS: Sets the number of columns on the screen. Takes precedence over the number of columns specified by the TERM variable.
EDITOR: The name of the editor (used for the v command).
HOME: Name of the user's home directory (used to find a lesskey file on UNIX systems).
INIT: Name of the user's init directory (used to find a lesskey file on OS/2 systems).
LANG: Language for determining the character set.
LC_CTYPE: Language for determining the character set.
LESS: Options that are passed to less(1) automatically.
LESSBINFMT: Format for displaying non-printable, non-control characters.
LESSCHARDEF: Defines a character set.
LESSCHARSET: Selects a predefined character set.
LESSCLOSE: Command line to invoke the (optional) input-postprocessor.
LESSECHO: Name of the lessecho program (default "lessecho"). The lessecho program is needed to expand metacharacters, such as * and ?, in file names on UNIX systems.
LESSEDIT: Editor prototype string (used for the v command). See PROMPTS.
LESSKEY: Name of the default lesskey(1) file.
LESSMETACHARS: List of characters that are considered "metacharacters" by the shell.
LESSMETAESCAPE: Prefix that less will add before each metacharacter in a command sent to the shell. If LESSMETAESCAPE is an empty string, commands containing metacharacters will not be passed to the shell.
LESSOPEN: Command line to invoke the (optional) input-preprocessor.
LESSSECURE: Runs less in "secure" mode. See SECURITY.
LESSSEPARATOR: String to be appended to a directory name in file-name completion.
LINES: Sets the number of lines on the screen. Takes precedence over the number of lines specified by the TERM variable.
PATH: User's search path (used to find a lesskey file on MS-DOS and OS/2 systems).
SHELL: The shell used to execute the ! command, as well as to expand file names.
TERM: The type of terminal on which less(1) is being run.
VISUAL: The name of the editor (used for the v command).

WARNINGS

The = command and prompts (unless changed by -P) report the line number of the line at the top of the screen but the byte and percent of the line at the bottom of the screen.

If the :e command is used to name more than one file, and one of the named files has been viewed previously, the new files can be entered into the list in an unexpected order.

On certain older terminals (the so-called "magic cookie" terminals), search highlighting will cause an erroneous display. On such terminals, search highlighting is disabled by default to prevent possible problems.

In certain cases, when search highlighting is enabled and a search pattern begins with a ^, more text than the matching string can be highlighted.

On some systems, setlocale(3) claims that ASCII characters 0 thru 31 are control characters rather than binary characters. This causes less(1) to treat some binary files as ordinary, non-binary files. To work around this problem, set the environment variable LESSCHARSET to "ascii" (or to whatever character set is appropriate).